Titanium and titanium-containing alloys are known in the art as possessing excellent strength to weight ratio, fracture toughness, corrosion resistance and biocompatibility; however, these materials are also characterized as having unsatisfactory wear performance. Thus, continued research in this area is oriented to improve the wear performance of titanium or titanium-containing alloys without adversely effecting the other physical properties of these materials.
It is well known in the art that titanium and titanium-containing alloys can be nitrided to form a hard surface layered material which has improved wear characteristics and fatigue crack initiation resistance.
U.S. Pat. No. 3,677,832 to Van Thyne et al. relates to a group of ternary or higher alloyed metals which consist essentially of Ti, at least one of Va, Be and Ta, and at least one of Mo and W. These alloys are then nitrided to cause surface hardening without any substantial chipping or brittleness. The nitrided alloys demonstrate improved wear and abrasion resistance.
U.S. Pat. No. 4,465,524 to Dearnaley et al. provides a workpiece of titanium or a titanium-containing alloy having a surface treated to improve its wear resistance. The surface of the titanium or its alloy are first coated with a layer of a metal such as Al, Co, Fe, Sn, Ni, Pt, Zn or Zr and then subjected to bombardment with light ion species.
The process of nitriding titanium or its alloys has led to increased applications for these materials. Such applications include tribological orthopaedic devices, gears, valves, pumps and the likes thereof.
In recent years, there have been several successful methods for producing a TiN surface layer on a titanium or a titanium-containing alloy in an attempt to improve the wear performance of these materials. These methods include reactive sputtering, physical vapor deposition, chemical vapor deposition, ion implantation and pulse implantation.
The first three methods are deposition processes which produce a discrete TiN film on the substrate whereas ion implantation is a physical process. Pulse ion implantation provides a three dimensional coverage but the method is depth limited and produces a fine distribution of TiN particles rather than a continuous layer. In addition to these undesirable results, the method requires high vacuum (in the order of 10.sup.-6 Torr) and a high energy accelerator (50-100 KeV).
Conventional ion nitriding is another method of producing TiN at the surface of titanium and titanium-containing alloys. Conventional ion nitriding is usually conducted at relatively high pressures of about 1 to 10 Torr and high temperatures of about 700.degree.-1100.degree. C. with the applied DC voltage ranging from 300-800V. This method is characterized by a low ionization efficiency and low particle energy. Ion nitriding of titanium or titanium-containing alloy has been found to produce a thin surface layer which is composed of cubic .delta.-TiN phase followed by a .epsilon.-Ti.sub.2 N layer and an interstial nitrogen diffusion zone in the adjacent .alpha.-Ti matrix: for example see A. Raveh, et al., Surface and Coatings Technology, Vol. 43/44 (1990), pgs. 744-755; A. Raveh, et al., Surface and Coatings Technology, Vol. 38 (1989), pgs. 339-351; A. Raveh, et al., Thin Solid Israel J. of Tech., Vol. 24 (1988), pgs 489-497; and E. S. Metin and O. T. Inal. Light Metal Age, October 1989, pgs. 26-30.
The method of ion nitriding typically employs a glow discharge source to produce an energetic flux of nitrogen ions and neutral species that heats the work piece, sputter cleans the surface, supplies active nitrogen and provides the energy for compound formation.
British Patent No. 2,190,100 relates to a forge, cast or sinitered titanium alloy and machine parts made therefrom the surface layers of which are treated at above 700.degree. C. in glow-discharge plasma. The resultant materials treated by such a process are characterized as having improved abrasion resistance. The surface layers are derived from a treatment gas containing small quantities (partial pressure 0.1 to 4 mbar) of nitrogen and, if necessary, carbon and/or oxygen.
Previous studies indicate that the growth of the nitride layer is controlled by a volume-diffusion process, thus the surface depth achieved by ion nitriding is proportional to the square root of time. Despite its potential success, conventional ion nitriding has the following disadvantages: (1) the method requires high temperature which makes processing of temperature sensitive materials difficult and (2) nitriding some materials is not feasible. Therefore, continued improvement in the area of ion nitriding is continually being sought in order to provide articles with enhanced wear and corrosion resistance. Such articles possessing these characteristics makes them suitable for use as orthopaedic implant devices and other applications or devices requiring resistance to wear and corrosion.